The present invention relates generally to power supplies, and more specifically to a power monitor circuit for a computer power supply.
With a prior art power monitor circuit as shown in FIG. 1, AC power-line voltage is supplied through an isolating transformer 10 to a rectifier-filter 11 in which it is converted to a DC voltage. One of the output terminals of the converter is coupled by way of a circuit breaker 12 to one input terminal of a voltage stabilizer 14 and the other outer terminal of the converter is coupled direct to the other input of the stabilizer. A short circuit 13 including a thyristor is connected across the input terminals of the stabilizer to provide a short-circuit path in response to a triggering signal supplied from the stabilizer if it encounters an overvoltage or overcurrent condition. When this occurs, circuit breaker 12 is triggered to mechanically latch this abnormal condition, giving a warning signal across DC alarm terminals 18. The output terminals of the stabilizer are coupled to a power controller 15 which supplies various control signals to a computer. The AC power-line voltage is also applied to a rectifier-filter 16 whose outputs are coupled to a low-voltage detector 17 to generate a warning signal across AC alarm terminals 19 if a voltage drop occurs in the AC power input.
However, prior art voltage stabilizer 14 has no ability to detect low DC voltages caused by failures other than overvoltages and overcurrents due to the fact that, if short circuit 13 is to be triggered in response to the detection of a DC low voltage condition, transitory events such as power-on states, AC-voltage drops and power outages are also undesirably detected as a power-line failure. The low voltage detection may be possible with the use of a separate inhibit circuit for disabling AC alarms which are generated at the instant the power monitor circuit is initially energized or at the instant a short-duration AC voltage drop occurs, but at some cost of complexity and additional hardware. In addition, the DC alarm signal must be maintained by mechanically latched contacts even though power controller 15 has become inactive following the occurrence of an overvoltage or overcurrent. Since the AC power line must be isolated from the short circuit that triggers the mechanical latch by the use of transformer 10, this adds to the overall cost and size of the power monitor circuit.